Plus, Simon Cowell reveals who he wants to win if Fifth Harmony doesn't.

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Inside "The X Factor" studio Wednesday night, all eyes were on finalists Tate Stevens, Carly Rose Sonenclar and the girls of Fifth Harmony--even Britney's mama, Lynne Spears, and the Jersey Shore cast were in the house to show their support.

But backstage, mentor L.A. Reid was the center of attention as he reflected on his impending exit from the Fox singing competition. "I'm very sad to leave," said the record exec. "I'm not happy. But I have responsibilities that I have to get back to.

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"[I] came into this knowing that it wasn't gonna become my life," continued L.A., who insisted he and Simon Cowell would continue to work together behind the scenes. "Simon and I are really good friends, and he knew at some point I needed to get back to work."

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What will he miss the most about the "X" gig? "The talent," he said without hesitation. "Watching people come in an unknown and walking away famous, [and] some of them walking away real stars--that's the greatest feeling in the world."

No wonder Simon cracked that L.A. was already "regretting his decision, trust me." He insisted he hadn't given any thought yet to who might replace L.A. on the panel next season: "It'd be crazy to think about it while the show's [still] on. I'll take a couple weeks off [after Thursday's finale], regroup, and wait for the calls to come in. I prefer that."

While Simon's naturally pulling for his remaining act Fifth Harmony to take the $5-million grand prize, he revealed 37-year-old blue-collar cowboy Tate would be his next choice. "Because of his age, you kind of think it'd be fairer to give him the money," Simon said. "But this is one of those nights where anyone could win, and I'd be really happy."

Tate wasn't feeling nearly so carefree. "I'm not gonna sleep tonight, I know that!" said the Missouri-based family man. "It's gonna be a long night."

Carly Rose was similarly anxious about tomorrow's potentially life-changing results, though she said her duet with LeAnn Rimes earlier in the evening had momentarily chilled her out. "Her style is very laid-back and natural," the 13-year-old phenom said admiringly of the 30-year-old singer, who began her own career in her early teens. "She doesn't think about it too much. She just sort of does what she feels in the moment, so I kind of took after her with that, and it helped me a lot."

Getting through the show's opening performance of Michael Jackson's "You Are Not Alone," a moving tribute to the victims of the tragedy in Newtown, Conn., was considerably harder for the singers. And Simon, who grew visibly teary while introducing the number. "We made the decision [to do the tribute] as soon as [the tragedy] happened," he said. "We were planning the show and saw it on the news, and I called the producers and said, 'We have to acknowledge what's happened in next week's show. It would be horrible to ignore it.'"

Even so, he admitted, "it was the hardest thing I've ever had to do on TV in my life."